It happens only in the tropics — that is, the latitudes between the Tropic of Cancer and the Tropic of Capricorn. Twice a year there — first when the sun is on its way north to the Tropic of Cancer, which it will reach at the summer solstice, then when it is on its way  back south to the Tropic of Capricorn, which it will reach at the winter solstice — the sun at noon reaches a point directly overhead and a vertical object will cast no shadow. In Hawaii, where I live, the two dates will be in May and July. And the word tropic means turning point.

Today on Oahu, the still point (terms zenith passage or Lahaina noon) was predicted for 12:28, plus or minus a few minutes on different parts of the island. I set up my vertical object at 9:00 in brilliant sun and started shooting. The brilliant sun persisted until just about noon, but then the clouds rolled in. However, with the camera facing approximately north:

Feel free to copy, but please credit me.